JP2005152114A - Mri method and mri apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a MRI apparatus which can simultaneously provide images with desired new contrasts such as a T1 emphasized image and a T2 emphasized image, and a MRI method. <P>SOLUTION: (1) Four types of pulse sequence diagrams (PSD) of a PSD for a first image with (TE and TR)=(TE1 and TR1), a PSD for a second image with (TE and TR)=(TE1 and TR2), a PSD for a third image with (TE and TR)=(TE2 and TR1), and a PSD for a fourth image with (TE and TR)=(TE2 and TR2) are prepared under a condition of (TE1<TE2 and TR1<TR2) (a step 11). (2) A magnet system of the MRI apparatus is driven following the above four PSD and MR signals at the time are collected (steps 12-15). (3) Four types of MR images S1-S4 are generated from the MR signals which are collected following each PSD (a step 16). (4) The mutual difference between the images (S3-S4)-(S1-S2) is acquired and the T1 and T2 emphasized images are obtained for the acquired difference image. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a magnetic resonance image processing (MRI) method for constructing an image using an MR signal detected using a magnetic resonance (MR) phenomenon, and an MRI apparatus for performing the method.
In particular, the present invention relates to an MRI method and an MRI apparatus for generating a useful contrast image from a plurality of different contrast images.

  In clinical practice using MR images, the contrast of an image of a test site of a subject (human body) is enhanced, and as such contrast enhancement, T1 enhancement that emphasizes a tissue having a short T1 value is performed. Basically, T2 emphasis emphasizing a tissue having a long T2 value, and proton density emphasis emphasizing a tissue having a high hydrogen density.

As a method for enhancing the contrast of such an image, for example, T1 enhancement in the spin echo method (SE), T2 enhancement in the first (fast) spin echo method (FSE), proton density enhancement in the FSE method, gradient (gradient) ) T2 ^* enhancement in echo method (GRE) and T2 / T1 enhancement in steady state free precession (SSFP) mode have been attempted.

However, the above-described method encounters the disadvantage that only an image having a contrast depending on only one piece of information can be obtained.
For example, even if there is a demand for emphasizing only a tissue having a long T1 value and a long T2 value, it cannot be realized by the conventional method.
Further, for example, even if only an organization having specific T1 value and T2 value is emphasized, it cannot be realized by the conventional method.

On the other hand, a calculation image processing method is known as a method for selecting only a specific tissue. The calculation image processing method is a method for obtaining T1 value and T2 value by calculation from several images such as T1 weighting, T2 weighting, proton density (PD) weighting.
However, this method encounters a problem that accurate calculation cannot be performed when image information in one pixel includes a plurality of types of tissues.
Further, this method is disadvantageous in that it is greatly affected by the calculation error due to noise and cannot be avoided.

  An object of the present invention is to provide an MRI apparatus and an MRI method capable of providing an image with a desired new contrast without adding complicated processing.

  According to the first aspect of the present invention, at least two pulse sequence databases (PSD) having different echo times (TE) and different repetition times (TR) are prepared, and the MRI apparatus is configured according to the two PSDs. The magnet system is driven, MR signals at that time are collected, an MR image is generated from the MR signals collected according to each PSD, and a tissue having a specific T1 value is emphasized for an image obtained by calculating a difference between two MR images. An MRI method is provided.

Specifically, a tissue having a long T1 value is emphasized for the obtained difference image.
Specifically, the tissue having a long T1 value and a long T2 value is emphasized for the obtained difference image.

According to a second aspect of the present invention, at least two pulse sequence databases (PSDs) having the same repetition time (TR) and different echo times (TE) are prepared,
Drive the magnet system of the MRI apparatus according to the two PSDs, collect MR signals at that time,
Generating first and second MR images from MR signals collected according to each PSD;
Obtaining a difference between two MR images and emphasizing a tissue having a specific T2 value from the obtained difference image;
An MRI method is provided.

According to the third aspect of the present invention, the following four kinds of pulse sequence databases (PSD) having different echo times (TE) and repetition times (TR) are prepared,
(1) The echo time of the first PSD is TE1, and the repetition time is TR1,
(2) The echo time of the second PSD is TE1, and the repetition time is TR2.
(3) The echo time of the third PSD is TE2, and the repetition time is TR1,
(4) The echo time of the fourth PSD is TE2, and the repetition time is TR2.
However, the first echo time TE1 <the second echo time TE2,
First repetition time TR1 <second repetition time TR2
Is,
Drive the magnet system of the MRI apparatus according to the four PSDs, collect MR signals at that time,
Generating first to fourth MR images from MR signals collected according to each PSD;
(Third image-fourth image)-(first image-second image) is obtained, and a tissue having a specific T1 value and a specific T2 value is emphasized for the obtained difference image.
An MRI method is provided.

  Preferably, when calculating the difference between the images, one image is multiplied by an erasing coefficient for erasing a specific tissue.

According to a fourth aspect of the present invention, there is provided an apparatus for performing the MRI method of the first aspect of the present invention, that is, an MRI apparatus.
The MRI apparatus is assumed to have the same echo time (TE) and drives the magnet system of the MRI apparatus according to at least two pulse sequence databases (PSDs) having different repetition times (TR), and the magnet system is driven. A collecting means for collecting the MR signals, an image generating means for generating at least two MR images from the collected MR signals, a difference between the two generated MR images, and a specific difference from the obtained difference image Image enhancement means for enhancing a tissue having a T1 value.

Specifically, the image enhancement unit obtains a difference image between the generated second image and the first image, and emphasizes a tissue having a long T1 value with respect to the obtained difference image.
More specifically, the image enhancement means emphasizes a tissue having a long T1 value and a long T2 value for the obtained difference image.

According to a fifth aspect of the present invention, there is provided an apparatus for performing the MRI method according to the second aspect of the present invention, that is, an MRI apparatus.
The MRI apparatus has the same repetition time (TR) and the following two PSDs having different echo times (TE), driving means for driving the magnet system of the MRI apparatus,
Collecting means for collecting MR signals when the magnet system is driven;
Image generating means for generating first and second MR images from the collected MR signals;
Image enhancement means for obtaining a difference between the generated second image and the first image and enhancing a tissue having a specific T2 value in the obtained difference image.

According to a sixth aspect of the present invention, there is provided an apparatus for performing the MRI method of the third aspect of the present invention, that is, an MRI apparatus.
The MRI apparatus has the following four kinds of pulse sequence databases (PSD) having different echo times (TE) and repetition times (TR), and hence driving means for driving the magnet system of the MRI apparatus,
(1) The echo time of the first PSD is TE1, and the repetition time is TR1,
(2) The echo time of the second PSD is TE1, and the repetition time is TR2.
(3) The echo time of the third PSD is TE2, and the repetition time is TR1,
(4) The echo time of the fourth PSD is TE2, and the repetition time is TR2.
However, the first echo time TE1 <the second echo time TE2,
First repetition time TR1 <second repetition time TR2
Is,
Collecting means for collecting MR signals when the magnet system is driven;
Image generating means for generating first to fourth MR images from the collected MR signals;
An image that obtains a difference between the following images, (third image-fourth image)-(first image-second image), and emphasizes a tissue having a specific T1 value and a specific T2 value with respect to the calculated difference image. And emphasis means.

  Preferably, in the image enhancement means, when calculating the difference between the images, one image is multiplied by an erasing coefficient for erasing a specific tissue.

In the present invention, a pulse sequence database (PSD) determined by an echo time TE and a repetition time TR,
(1) Same echo time and different repetition time, or
(2) Same echo time with different echo time, or
(3) A plurality of PSDs having different echo times and different repetition times are prepared, the magnet system is driven for those PSDs, MR signals at that time are collected, and the difference between the obtained images is taken to prepare the prepared PSDs. Depending on the TR or TE, an enhanced image of the desired tissue is obtained.
Preferably, when calculating the difference between the images, a specific tissue is erased from the image by multiplying one image by a specific tissue erasure coefficient.

  Note that when performing PSD including the echo time and the repetition time, for example, a spin echo (SE) method, a fast spin echo (FSE), or a gradient echo method can be used.

In the present invention, a plurality of different PSDs are prepared, the magnet system is driven for those PSDs, MR signals at that time are collected, and the difference between the obtained images is taken to obtain a repetition time TR of the prepared PSDs or A desired enhanced image can be obtained according to the echo time TE.
That is, according to the present invention, an image having one of the echo time TE and the repetition time TR that are the same and the other being different, or an image having two or more contrasts that are different from each other is synthesized and processed. Only tissues with T2 values can be emphasized.

For example, if specific T1 is emphasized, moisture and fat can be erased from the image at the same time. According to the present invention, since specific T1 can be trained, there is an advantage that moisture and fat can be erased simultaneously.
Further, when a specific T2 is emphasized, it is possible to select and view a desired tissue. Further, according to the present invention, since a specific T2 value can be adjusted, it is possible to select a tissue that the user wants to see.

  Furthermore, the signal intensity of fat can be reduced by synthesizing (difference) an image having a sufficiently long TR and an image having a short TR.

  Preferably, when two or more images are combined, a difference (combination) can be performed by multiplying an appropriate coefficient instead of a simple difference. As such a coefficient, a ratio of two images to be synthesized can be used, or a fixed value can be used.

  In the present invention, since fat suppression can be performed using each data, fat suppression is possible even with a magnet system and a low magnetic field system in which the magnetic field is not uniform.

  Similarly, the present invention has an advantage that the contrast can be easily calculated as compared with IR by the FSE method in which fat suppression is performed using nuclear data.

  Unlike the conventional T1 and T2 calculation images, the present invention can obtain the above-described effect even when a plurality of tissues exist in one pixel.

  The present invention only needs to be set so that the desired enhancement of PSD is realized, and there is no need to change the control means, data processing means, etc. in the MRI apparatus.

  An MRI apparatus according to an embodiment of the present invention and a signal processing method in the MRI apparatus will be described.

Configuration of the MRI apparatus will be described first configuration example of the MRI apparatus of the present invention.
FIG. 1 is a configuration diagram of an MRI apparatus as a first embodiment of the present invention.
The MRI apparatus illustrated in FIG. 1 includes a magnet system 100, a data collection unit 150, an RF coil driving unit 140, a gradient coil driving unit 130, a control unit 160, a data processing unit 170, a display unit 180, And an operation unit 190.

The magnet system 100 includes a main magnetic field coil unit 102, a gradient coil unit 106, and an RF coil unit 108, and is installed in a scan room that is shielded against electromagnetic waves and magnetism.
The main magnetic field coil unit 102, the gradient coil unit 106, and the RF coil unit 108 are configured in a substantially cylindrical shape, and the human body 300 as the subject is placed in the cradle 500 in the internal space (bore) of the magnet system 100. It is placed and moved so that it can be carried in and out along with the cradle 500 by a transport means (not shown) according to the region to be examined.

The main magnetic field coil unit 102 forms a static magnetic field in the bore of the magnet system 100. The direction of the static magnetic field is substantially parallel to the body axis direction of the human body 300 as the subject, and forms a horizontal magnetic field.
In the present embodiment, an example in which the main magnetic field coil section 102 is configured by a magnet such as a superconducting magnet is shown.

  The gradient coil drive unit 130 drives the gradient coil unit 106 under the control of the control unit 160 to give a gradient (gradient) to the static magnetic field strength formed in the magnet system 100. There are three types of gradient magnetic fields generated by the gradient coil unit 106: a slice gradient magnetic field, a readout gradient magnetic field, and a phase encoding gradient magnetic field, and the gradient coil unit 106 generates these three types of gradient magnetic fields. It has a seed gradient coil.

The RF coil unit 108 includes a transmission coil and a reception coil. Each of the transmission coil and the reception coil may be provided with two dedicated coils, or may be provided with only one coil in common. In this specification, for convenience, the RF coil unit 108 is treated as including both a transmission RF coil and a reception RF coil.
The RF coil driving unit 140 drives (excites) the transmission coil (transmission RF coil) of the RF coil unit 108 under the control of the data processing unit 170, and covers the static magnetic field space of the magnet system 100. A high frequency magnetic field for exciting spins in the human body 300 as a specimen is formed. A magnetic resonance (MR) signal that is an electromagnetic wave generated by a spin excited by the receiving coil (receiving RF coil) of the RF coil unit 108 is detected.

  For example, in the steady state free precession (SSFP) state, the reception RF coil unit 108 (reception RF coil) includes a free induction decay (FID) signal, a spin echo (SE) signal, and an excitation echo (STE). An MR signal on which a signal or the like is superimposed is detected.

  Under the control of the control unit 160, the data collection unit 150 inputs (collects) MR signals detected by the reception RF coil and outputs them to the data processing unit 170.

The data processing unit 170 has a computer, and cooperates with the control unit 160 in accordance with various programs stored in the memory of the computer to create a pulse sequence database (PSD) set by a method described in detail below. Based on the MR signal collection process, the process of generating an image from the collected MR signal, the difference between the generated images is calculated, and various enhancement processes are performed on the difference image. The data processing unit 170 performs various operations related to the MRI process.
That is, for example, the data processing unit 170 stores the MR signals collected by the data collection unit 150 in a memory in which the data space of the two-dimensional Fourier space in the data processing unit 170 is defined, and the MR signals stored in the memory Is used to perform various signal processing including perfusion image processing using, for example, maximum intensity projection (MIP).
The data processing unit 170 displays the processing result on the display unit 180 as an MRI image of the subject.

The control unit 160 cooperates with the data processing unit 170 to control the gradient coil driving unit 130, the RF coil driving unit 140, and the data collecting unit 150 according to the set PSD, and to examine the human body 300 that is the subject. Control for imaging the region is performed.
In other words, in the present invention, the control unit 160 performs the processing described below based on the set PSD in cooperation with the data processing unit 170. In the present embodiment, the PSD is set by the operator from the operation unit 190 and is stored in the memory of the control unit 160 via the data processing unit 170.

The operation unit 190 is used by a doctor, engineer, or the like (hereinafter referred to as an operator) who uses the MRI apparatus to instruct a desired operation process. The data processing unit 170 and the control unit 160 cooperate to process the content instructed by the operation unit 190.
In the present embodiment, the operator sets the PSD in the memory of the control unit 160 via the operation unit 190.

  The controller 160 energizes (excites) various magnetic field generating coils in the magnet system 100 according to PSD. Examples of the excitation will be described below.

General operation of the MRI apparatus An operator who operates the MRI apparatus instructs a desired MRI operation from the operation unit 190. Prior to the MRI operation, the operator sets a desired PSD in the control unit 160.

  The data processing unit 170, together with the control unit 160 in accordance with an instruction from the operation unit 190, includes a slice gradient magnetic field in the static magnetic field generated by the main magnetic field coil unit 102 via the gradient coil driving unit 130 according to the set PSD. Generate a readout gradient magnetic field, a phase encoding gradient magnetic field, and the like. Further, the data processing unit 170 excites the transmission coil via the RF coil driving unit 140 together with the control unit 160 in accordance with an instruction from the operation unit 190, and enters the subject 300 in the static magnetic field space of the magnet system 100. A high-frequency magnetic field is formed to excite spins.

  The data processing unit 170 and the control unit 160 drive a transport unit (not shown) according to the test site of the human body 300 that is the test subject to move the cradle 500 into the bore of the magnet system 100.

For example, a doctor injects (intravenously) an MR contrast agent, for example, a gadolinium (Gd) compound contrast agent, intravenously into the human body 300 as the subject.
Gd does not appear directly in the MR image, but promotes the relaxation of hydrogen protons in the tissue, and its presence is indirectly reflected in the display unit 180 as an MR image by processing in the data processing unit 170. It will be reflected. Therefore, the MR signal detected after injecting the contrast agent indirectly indicates the position and concentration of the contrast agent.

The receiving RF coil continuously detects MR signals (echo signals) that are electromagnetic waves generated by the excited spins.
The data collection unit 150 continuously inputs MR signals continuously detected by the receiving RF coil and continuously outputs them to the data processing unit 170.
The data processing unit 170 operates various programs related to the MRI processing stored in the memory of the computer that constitutes the data processing unit 170, and the MR signals collected by the data collection unit 150 are two-dimensionally stored in the data processing unit 170. Various kinds of signal processing such as performing perfusion image processing using a method such as maximum luminance projection (MIP) using the MR signal stored in the memory in which the Fourier space data space is defined. I do. The data processing unit 170 displays the perfusion image on the display unit 180.

Hereinafter, the processing in the control unit 160 will be mainly described.
Since T1 emphasis and T2 emphasis are defined by the PSD, the operator sets a desired PSD in the control unit 160 or the data processing unit 170 in advance using the operation unit 190. The controller 160 performs a driving process of the magnet system 100 based on the set PSD.

Basic contents of MRI related to the present invention Before describing the details of the MRI apparatus and MRI method of the present invention, the contents related to the present invention will be described among the techniques related to MRI.
(1) TR is a repetition (repetition) time. The repetition time TR is a time interval at which an RF pulse used for obtaining a resonance signal is output, and determines the amount of longitudinal relaxation (T1 relaxation, spin / lattice relaxation).
(2) TE refers to echo (delay) time (hereinafter referred to as echo time). The echo time TE is the time from when the first RF pulse for exciting the spin is output until the echo signal is obtained. The echo time TE is the degree of dispersion of the transverse magnetization spin (lateral relaxation, T2 relaxation, spin spin Mitigation).
(3) T1 refers to the longitudinal relaxation time (or spin lattice relaxation time).
(4) T2 means transverse relaxation time (or spin / spin relaxation time).
(5) T2 ^* is an exponential time constant representing the attenuation of a free induction decay (FID) signal in the gradient echo method (e ^{−t / T} ₂ ^* ).

  The longitudinal relaxation time T1 and the transverse relaxation time T2 are characteristics unique to the tissue, and T1 and T2 are determined for each tissue. T1 and T2 of typical organizations are exemplified below.

Lateral relaxation time T2
The transverse relaxation time T2 of the tissue is characterized by the speed of phase dispersion of spins of hydrogen nuclei in the tissue. If the phase dispersion is early, T2 becomes short, and if the phase dispersion is slow, T2 becomes long.
The spin-spin interaction between hydrogen protons is very small due to the structure of water molecule H—O—H and its thinness, and therefore phase dispersion proceeds at a much slower rate in water than in other tissues. . Therefore, the T2 relaxation time of water is long.
The molecular structure of the solid is symmetric with pure water, and the interaction between hydrogen protons frequently occurs with a very close structure. Since the phase dispersion proceeds fast due to many spin-spin interactions, the T2 time of a solid is short.
The structure of fat and proteinaceous substances is such that the phase dispersion is slower than solid and faster than water, so the T2 time of fat and proteinaceous substances is located between water and solids.

Longitudinal relaxation time T1
The longitudinal relaxation time T1 of the tissue is affected by whether or not hydrogen nuclei in the tissue can exchange energy with the surrounding lattice. It is known that the highest rate of energy application occurs when the free movement of protons reaches the Larmor frequency. In other words, the precession frequency of hydrogen protons is 42.6 MHz with a magnetic field of 1 Tesla. However, the natural motion frequency depends on the physical state of the tissue, and is greatly influenced by the bonded atoms and nearby atoms.
In small molecules of water, for example, hydrogen protons have a relatively high natural frequency compared to hydrogen protons in solids. And the natural motion frequency of hydrogen protons in water is much higher than the hydrogen Larmor frequency. Therefore, T1 time of water is long.
The natural motion frequency of hydrogen protons in solids is lower than in water. The natural motion frequency of hydrogen protons in solids is somewhat lower than the hydrogen Larmor frequency. Thus, the T1 time for solids is somewhat shorter than the T1 time for water.
Fatty hydrogen protons have approximately the same natural motion frequency as the Larmor frequency used in MRI. This is because the rotational motion frequency of carbon around the terminal carbon-carbon bond in fat is close to the Larmor frequency, which is a result of increasing the energy transfer between protons and lattices, and shortens the T1 time.
The proteinaceous substance T1 is located between the solid and water.

FIG. 2A shows a T1 recovery curve for fat, water, and solid, and FIG. 2B shows a T2 decay curve for fat, water, and solid.
2 (A) and 2 (B), it can be seen that fat has the shortest T1 and shows the steepest T1 recovery curve. Similarly, it can be seen that the proteinaceous liquid exhibits a short T1. It can be seen that water has the longest T1 and exhibits the slowest T1 recovery curve. It can be seen that the solid has an intermediate T1.

As described above, the longitudinal relaxation time T1 is defined by the repetition time TR, and a long TR reduces the influence of T1, and a short TR enhances the influence of T1 (enhances contrast). On the other hand, the transverse relaxation time T2 is defined by the echo (delay) time TE, a short TE reduces the effect of T2 (T2 ^* ), and a long TE enhances the effect of T2 (T2 ^* ) (enhances contrast). .
Each of the T1-weighted image and the T2-weighted image is an image in which a certain tissue is emphasized.

The T1-weighted image is, for example, an image captured with a short repetition time TR in the spin echo (SE) method. When T1 enhancement is performed in the SE method, a short TE (for example, several ms) and a short TR (for example, 300 to 500 ms) can be set to generate an image of a tissue having a longer T1 value with high luminance. it can.
FIG. 3 illustrates an example of the SE method.

The T2-weighted image is, for example, an image captured at a long repetition time TR in the spin echo (SE) method or the fast spin echo (FSE) method. Since the echo signal generated in this case follows T2 attenuation, such an image is called a T2-weighted image.
When a T2-weighted image is obtained by the SE method, the next RF pulse is applied after the longitudinal magnetization has sufficiently recovered, so that the repetition time TR becomes longer. If the repetition time TR is long, there is a problem that the imaging time becomes very long. To solve this problem, the FSE method is used.

FIG. 4 illustrates an example of the FSE method.
The FSE method uses a single 90 ° pulse for excitation and a plurality of subsequent 180 ° pulses for convergence to generate a large number of spin echoes, which are separately used as phase encode signals. An imaging method that achieves a high speed.
When T2 emphasis is performed in the FSE method, a long TE (for example, 100 to 200 ms) and a long TR (for example, 2000 to 5000 ms) can be set to generate an image of a tissue having a longer T2 value with high luminance. it can.
When proton density emphasis is performed in the FSE method, a short TE (for example, several ms) and a long TR (for example, 2000 to 5000 ms) can be set to generate an image of a tissue with a higher hydrogen density with high brightness. it can.

T2 ^* and weighted image, since the relaxation accordance T2 ^* decay, such as free induction decay (FID) occurs in the gradient echo (GRE) method is called an image obtained by such a method and T2 ^* weighted images.
When T2 ^* enhancement is performed in the GRE method, by setting a long TE (for example, 20 ms) and a long TR (for example, several hundred ms) as the GRE method, an image of a tissue having a longer T2 ^* value can be displayed with high brightness. Can be generated.

  When T2 / T1 weighting is performed in the steady state free precession (SSFP) mode, a short TE (for example, several ms) for the gradient echo method (GRE method) and a short TR (for example, 10 ms) for the GRE method. ) Can generate an image of a tissue having a high ratio T2 / T1 with high luminance.

Specific example repetition time TR of PSD is defined by PSD. In other words, the operator of the MRI apparatus is defined by the set PSD based on the above-described knowledge and based on the desired emphasis.
5A to 5E show specific PSD examples in the SSFP mode.
Such PSD is set in the control unit 160 described with reference to FIG.

FIG. 5A is a waveform diagram of the frequency of the excitation signal applied to the transmission RF coil. Between 1TR (between cycles), continuous RF signals are repeated under the control of the control unit 160. It is applied to the transmitting RF coil at a period of time TR. As described above, the gradient coil drive unit 130 drives the gradient coil unit 106 under the control of the control unit 160 to give a gradient (gradient) to the static magnetic field strength formed in the magnet system 100. There are three types of gradient magnetic fields generated by the gradient coil unit 106: a slice gradient magnetic field, a readout gradient magnetic field, and a phase encode gradient magnetic field, and the gradient coil unit 106 generates three types of gradients that generate these three types of gradient magnetic fields. It has a coil.
FIG. 5B is a waveform diagram of a slice (SLICE) pulse for generating the slice gradient magnetic field.
FIG. 5C is a waveform diagram of a lead pulse RD for generating the readout gradient magnetic field.
FIG. 5D is a waveform diagram of a warp pulse for generating the phase encoding magnetic field, and the warp pulse also has a central warp pulse WW0 between 1TR (between cycles) as illustrated by a solid line. The waveform is such that the integration of the upper and lower signals is zero. For example, the first warp pulse WP1 has a vertically symmetrical waveform. The same applies to the second warp pulse WP2 and the third warp pulse WP3.
FIG. 5E is a waveform diagram of an echo signal detected by the receiving RF coil as an MR signal. The data processing unit 170 receives the MR signal from the receiving RF coil, generates an MRI image MRIMAGE of the subject, and displays it on the display unit 180.

Examples of enhancement processing of the present invention PSDs used in the present invention include, for example, the PSD for the spin echo (SE) method illustrated in FIG. 3 and the fast spin echo (FSE) method illustrated in FIGS. 4 and 5. PSD, PSD for gradient echo (GRE) method, PSD for steady state free precession (SSFP) method, etc. are used.

First Embodiment, T1 Emphasis First, T1 emphasis will be described as a first embodiment of the emphasis processing of the present invention.
For example, the signal intensity S of the MR signal in the SE method or the FSE method is given by the following formula 1.

  Accordingly, MR signals (echo signals) used for image generation with the same echo time TE and different repetition times TR, that is, the first and second repetition times TR1 and TR2, have the following signal intensities S1 and S2. However, TR1 <TR2.

  When the difference (S1 to S2) between the signal strengths S1 and S2 of the two MR signals in the SE method or the FSE method is obtained, the following Expression 3 is obtained. However, (S2-S1) is illustrated as an example of the difference.

  When Equation 3 regarding the difference in signal intensity is differentiated by the T1 value to obtain the maximum value of the T1 value, the maximum value is expressed by the following Equation 4.

  For example, in Equation 4, when TR1 = 300 ms and TR2 = 1000 ms, as illustrated in FIG. 6A, the tissue of T1 = 581 ms is most emphasized, and the signal strength of other tissues increases as T1 moves away from 581 ms. descend. Therefore, an image in which the tissue of T1 = 581 ms is most emphasized can be obtained.

  In the MRI apparatus according to the first embodiment of the present invention, for example, the following processing shown in the flowchart of FIG. 7 is performed.

(1) Step 1
The operator prepares the following two PSDs having the same echo time TE and different first and second repetition times TR1 and TR2, and sets them in the control unit 160 via the operation unit 190.
The operation unit 190 corresponds to means for preparing a PSD in the present invention.
(A) PSD for first image (TE, TR) = (TE, TR1),
(B) PSD for second image (TE, TR) = (TE, TR2),
However, the first repetition time TR1 <the second repetition time TR2.

Note that the echo time and the first repetition time in the first image PSD used to capture the first image are shown as first image PSD (TE, TR) = (TE, TR1), and the second image. The echo time and the second repetition time in the second image PSD used to capture the image are shown as second image PSD (TE, TR) = (TE, TR2).
When implementing such PSD, for example, a spin echo (SE) method, a fast spin echo (FSE), a gradient echo method, an SSFP method, or the like can be used.

(2) Step 2
In the control unit 160, the magnet system 100 is excited with the first PSD of the first repetition time TR1, and the data collection unit 150 collects MR signals.
The control unit 160 corresponds to a means for driving the magnet system of the present invention, and the data collection unit 150 corresponds to a data collection means of the present invention.

(3) Step 3
In the control unit 160, the magnet system 100 is excited again with the second PSD of the second repetition time TR2, and the MR signal is collected by the data collecting unit 150.

(4) Step 4
The data processing unit 170 generates two images a and b from the two MR signals collected by the data collecting unit 150.
The data processing unit 170 corresponds to the image generation unit of the present invention.

(5) Step 5
The data processing unit 170 takes the difference between the two images a and b, and generates an image that emphasizes a tissue having a specific T1 value for the obtained difference image. The result is displayed on the operation unit 190, for example.
The data processing unit 170 corresponds to the emphasized image generation unit of the present invention.

Second Embodiment, T2 Enhancement T2 enhancement will be described as a second embodiment of enhancement processing of the present invention.
A specific T2 value can also be emphasized by the same method as in the first embodiment.
When two PSDs having the same repetition time TR and different echo times TE, ie, the first and second echo times TE1 and TE1 are different (provided that TE1 <TE2), the different echo times TE1 and TE1 are obtained. The signal strengths Sa and Sb represented by the following formula 5 are obtained.

  When the difference (Sa to Sb) between the signal intensities Sa and Sb of the two MR signals is obtained, the following Expression 3 is obtained. However, (Sa-Sb) is illustrated as an example of the difference (thus, TE1 <TE1).

  When the maximum value of T2 value is obtained by differentiating Equation 6 by T2 value, the following Equation 7 is obtained.

  For example, when TE1 = 50 ms and TE1 = 300 ms, as illustrated in FIG. 6B, the tissue of T2 = 150 ms is most emphasized, and the signal strength of the other tissues decreases as T2 moves away from 150 ms. Therefore, an image in which the tissue of T2 = 150 ms is most emphasized can be obtained.

Third Embodiment, General Enhancement In the third embodiment of the enhancement processing of the present invention, an image that emphasizes a tissue having only specific T1, T2 values is preferably generated from two contrasts.
As the PSD used in the third embodiment, for example, a PSD for the SE method or a PSD for the FSE method is used.
In the third embodiment, in order to generate an image having a specific contrast, images having different TR and TE are measured a plurality of times. Thereafter, only the tissues having specific T1 values and T2 values obtained by synthesizing those images are emphasized.
For example, the contrast of the SE method is generally expressed by the following formula.

  In this method, it is not necessary that n, that is, the type of tissue existing in one pixel is 1 (n = 1), and calculation can be performed without problems even if a plurality of tissues exist (mixed) in one pixel. In this respect, the present invention is superior to the conventional method.

A third embodiment will be described with reference to FIG.
FIG. 8, Step 11: The operator prepares the following four types of PSD and sets them in the control unit 160.
(1) PSD1 for first image (TE, TR) = (TE1, TR1),
(2) PSD2 for second image (TE, TR) = (TE1, TR2),
(3) PSD3 for third image (TE, TR) = (TE2, TR1),
(4) PSD4 for fourth image (TE, TR) = (TE2, TR2)
However, condition (TE1 <TE2, TR1 <TR2).

Steps 12-15: The control unit 160 and the data processing unit 170 sequentially perform the first to fourth PSD1 to PSD4.
That is, for example, the control unit 160 excites (drives) the magnet system 100 based on PSD1, and the data processing unit 170 collects the resulting MR signals. This process is performed for PSD2 to PSD4.

  Step 16: The data processing unit 170 generates the images 1 to 4 from the MR signals collected based on the PSDs 1 to 4.

  Step 17: The data processing unit 170 calculates (image 3 -image 4)-(image 1 -image 2). As a result, the following contrast is obtained.

  When the above equation 9 is partially differentiated with respect to T1 and T2, the maximum values of T1 and T2 are obtained. As the distance from the local maximum values of T1 and T2 increases, the signal strength decreases. As a result, a tissue having a T1 value and a T2 value can be emphasized.

Fourth Embodiment In the fourth embodiment of the present invention, for example, as a method of erasing a specific tissue, for example, a fat signal (image), an image obtained by PSD using a sufficiently long TR is used. By taking a difference from an image obtained by PSD using a short TR, fat signals can be suppressed.
In the case of emphasizing a specific T1, the difference between images at different repetition times TR1 and TR2 is a contrast represented by the following equation.

  In this case, if TR2 is sufficiently long, for example, in the case of TRb of about 2000 to 4000 ms, exp (−TR2 / T1n) becomes almost 0, and can be approximated by the following equation (11).

From Equation 11, it can be seen that both T1 and T2 can be emphasized in a long organization.
That is, since fat has T1 = 200 ms and T2 = 100 ms, signal strength tends to decrease. On the contrary, both T1 and T2 are long, and for example, tissues such as tumors, blood vessels, and cerebrospinal fluids with T1> 1000 ms and T2> 100 ms can be emphasized.

Fifth Embodiment In the above-described embodiment, when the difference between two images is taken, a simple difference is taken, but subtraction can also be performed by multiplying some coefficient. If the coefficient is a, for example, the following equation is obtained.

The coefficient a may be fixed.
Also, when it is desired to erase a specific tissue, for example, fat (fat), if the T1 value of fat is T1f, fat erase so that a = exp (−TR1 / T1f) / exp (−TR2 / T1f) If the coefficient a is optimized, the fat image can be efficiently deleted. The operator may adjust the fat erasure coefficient a.
In the above-described example, fat is described. However, it is also possible to erase images of other specific tissues. In that case, an erasing coefficient suitable for erasing the specific tissue is used.

Other embodiments The above method has been described for the case where the SE method, the FSE method, etc. are used. However, the present invention is not limited to the SE method, the FSE method, etc. For example, GRASS, Even if images with different contrasts such as the GRASS and SSFP methods are synthesized, the same result as in the above-described embodiment of the present invention can be obtained.

FIG. 1 is a configuration diagram of an MRI apparatus as a first embodiment of the present invention. FIG. 2A is a graph showing a T1 recovery curve of main tissues, and FIG. 2B is a graph showing a T2 attenuation curve of main tissues. FIG. 3 is a graph illustrating the SE method. FIG. 4 is a graph illustrating the FSE method. FIGS. 5A to 5E are graphs illustrating an example of PSD of the FSE method. FIG. 6A is a graph illustrating T1 weighting, and FIG. 6B is a graph illustrating T2 weighting. FIG. 7 is a first flowchart showing a control process performed by the MRI apparatus according to the first embodiment of this invention. FIG. 8 is a second flowchart showing a control process performed by the MRI apparatus according to the second embodiment of this invention.

Explanation of symbols

100 ・ ・ Magnet system
102 .. Main magnetic field coil section, 106 .. Gradient coil section
108 .. RF coil unit 130 .. Gradient coil drive unit 140 .. RF coil drive unit 150 .. Data collection unit 160 .. Control unit 170 .. Data processing unit 180 .. Display unit 190. -Subject, 500-Cradle

Claims (12)

Preparing at least two pulse sequence databases (PSD) with different echo times (TE) and different repetition times (TR);
Drive the magnet system of the MRI apparatus according to the two PSDs, collect MR signals at that time,
Generating MR images from MR signals collected according to each PSD;
Emphasize a tissue having a specific T1 value for an image obtained by calculating a difference between two MR images.
MRI method. Emphasize a tissue having a long T1 value for the obtained difference image;
The MRI method according to claim 1. Emphasize a tissue having a long T1 value and a long T2 value for the obtained difference image;
The MRI method according to claim 1. Preparing at least two pulse sequence databases (PSDs) having the same repetition time (TR) and different echo times (TE);
Drive the magnet system of the MRI apparatus according to the two PSDs, collect MR signals at that time,
Generating first and second MR images from MR signals collected according to each PSD;
Obtaining a difference between two MR images and emphasizing a tissue having a specific T2 value from the obtained difference image;
MRI method. Preparing four pulse sequence databases (PSDs) in combination of first and second echo times (TE) and first and second repetition times (TR);
(1) The echo time of the first PSD is TE1, and the repetition time is TR1,
(2) The echo time of the second PSD is TE1, and the repetition time is TR2.
(3) The echo time of the third PSD is TE2, and the repetition time is TR1,
(4) The echo time of the fourth PSD is TE2, and the repetition time is TR2.
However, the first echo time TE1 <the second echo time TE2,
First repetition time TR1 <second repetition time TR2
Is,
Drive the magnet system of the MRI apparatus according to the four PSDs, collect MR signals at that time,
Generating first to fourth MR images from MR signals collected according to each PSD;
(Third image-fourth image)-(first image-second image) is obtained, and a tissue having a specific T1 value and a specific T2 value is emphasized for the obtained difference image.
MRI method. When calculating the difference between the images, one image is multiplied by an erasing coefficient for erasing a specific tissue.
The MRI method according to claim 1. Driving means for driving the magnet system of the MRI apparatus according to at least two pulse sequence databases (PSDs) that have the same echo time (TE) and different repetition times (TR);
Collecting means for collecting MR signals when the magnet system is driven;
Image generating means for generating at least two MR images from the collected MR signals;
Image enhancement means for obtaining a difference between the generated two MR images and enhancing a tissue having a specific T1 value from the obtained difference image.
MRI equipment. The image enhancement means emphasizes a tissue having a long T1 value with respect to the obtained difference image;
The MRI apparatus according to claim 7. The image enhancement means emphasizes a tissue having a long T1 value and a long T2 value with respect to the obtained difference image;
The MRI apparatus according to claim 7. Driving means for driving the magnet system of the MRI apparatus according to two PSDs having the same repetition time (TR) and different echo times (TE);
Collecting means for collecting MR signals when the magnet system is driven;
Image generating means for generating first and second MR images from the collected MR signals;
An MRI apparatus comprising: an image enhancement unit that obtains a difference between the generated second image and the first image and emphasizes a tissue having a specific T2 value in the obtained difference image. Driving means for driving the magnet system of the MRI apparatus according to the following four kinds of pulse sequence databases (PSD) in combination of the first and second echo times (TE) and the first and second repetition times (TR): ,
(1) The echo time of the first PSD is TE1, and the repetition time is TR1,
(2) The echo time of the second PSD is TE1, and the repetition time is TR2.
(3) The echo time of the third PSD is TE2, and the repetition time is TR1,
(4) The echo time of the fourth PSD is TE2, and the repetition time is TR2.
However, the first echo time TE1 <the second echo time TE2,
First repetition time TR1 <second repetition time TR2
Is,
Collecting means for collecting MR signals when the magnet system is driven;
Image generating means for generating first to fourth MR images from the collected MR signals;
An image that obtains a difference between the following images, (third image-fourth image)-(first image-second image), and emphasizes a tissue having a specific T1 value and a specific T2 value with respect to the calculated difference image. An MRI apparatus comprising: emphasis means. In the image enhancement means, when calculating the difference between the images, one image is multiplied by an erasing coefficient for erasing a specific tissue.
The MRI apparatus according to claim 7.

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